The so-called "eddy current method" for object examination has been in widespread use for many years and involves the generation of eddy currents in an object under testing and determination of defects therein on the basis of detection of such eddy currents and their characteristics.
Eddy current evaluation of magnetic materials is dominantly affected by permeability variations in the object. In various test instances, permeability inhomogeneity of the object gives rise to a sufficiently high noise level to so lessen the "signal-to-noise ratio" as to undermine the efficacy of the testing. The art has, however, long recognized a solution to this problem for encircling coil inspection evaluations, namely, to DC (direct current) magnetically saturate the object such that the detection apparatus sees only a constant or unit permeability.
Situations arise, however, where DC magnetic saturation of the test object is not practical, for example, in the spinning probe inspection of large-diameter steel bar material and thick-walled pipes. In those instances, eddy current testing can be successfully practiced without DC magnetic saturation of the object. However, work surface finishes therein become a problematic factor. Thus, unevenness or surface roughness can give rise to noise generation of greater adverse consequence to signal-to-noise ratio than permeability inhomogeneity.
The art has likewise addressed the surface roughness problem. The eddy current results encompass flaw signals indicative of a defect in the object and noise signals arising from surface roughness. The solution, although of limited applicability, involves selection of an inspection frequency that allows for phase discrimination, i.e., the operator selects a test frequency which maximizes the polar phase difference between the flaw signals and the noise signals so as to increase the signal-to-noise ratio. Cold rolled materials generally have better surface finishes than hot rolled materials and the phase discrimination practice is more beneficial in application to the former materials. In the case of hot rolled materials, the degree of surface roughness can generate excessive polar noise, such that many smaller-depth defects cannot be detected at any phase relationship.
Difficulty accordingly attends eddy current examination of hot rolled materials. If they are nonmagnetic, the DC saturation practice is not applicable and if surface roughness is excessive, eddy current testing is limited. Where the hot rolled material is magnetic, but saturation is not practical as for spinning probe inspection, the same ineffectiveness of eddy current practice is at hand if surface roughness is excessive. In summary, the spinning probe eddy current practice as presently known is seen as usable effectively in examining magnetic hot rolled materials that do not have excessive surface roughness.
AC flux leakage, another inspection method, was introduced a few years ago primarily for testing hot rolled steel materials. The basic principle for AC flux leakage also relies on the skin effect and similarly magnetizes the surface region of the material in order to be able to generate leakage flux in the presence of a defect. Additionally, the magnetization force has to be applied normal to a defect in order to create a leakage flux and subsequently detect the defect.
Examples of the AC flux leakage method, particularly in its application to hot rolled materials, are seen, for example, in U.S. Pat. Nos. 5,023,550 and 4,297,636. The former patent particularly notes the inefficacy of the eddy current method for examination of hot rolled materials and looks to examination thereof by the AC flux leakage method.